1. Field of the Invention
The present invention relates to a method of producing a diamond film and a diamond film produced by the method.
2. Description of the Related Art
Recently, use of a diamond film formed on a substrate has been commonly considered by making use of peculiar characteristics of diamond. Diamond has been used for a mask member for exposure in lithography technique used in the manufacture of semiconductor devices, a substrate for a surface acoustic wave (SAW) device, or a grinding and polishing tool, for example.
Since diamond is excellent in Young's modulus, etching resistance, high energy ray radiation resistance, or the like, interest has been shown towards diamond to utilize it for a mask membrane for lithography with X-ray or electron beam, which can form ultrafine pattern of 100 nm or less.
As to a method of producing a diamond film, there have been known methods using DC arc discharge, DC glow discharge, combustion flame, high frequency (radio-frequency), microwave, hot-filament, or the like. Among them, the microwave CVD method and hot-filament CVD method are commonly used, because each method can form the film having a large area and good crystallinity. When forming a film of a vapor phase synthetic diamond by the above CVD methods, a mixed gas, in which a carbon contained gas such as methane, ethylene, acetylene or carbon monoxide in general is diluted by hydrogen gas, is commonly used as a source gas. However, when this hydrogen-diluted carbon contained gas is used as a source gas for vapor phase reaction, the value of electric resistance of the obtained diamond film is determined, with the result that it shows a range of 109 to 1015 Ω·cm.
If a diamond film is used for a mask membrane for lithography, particularly for lithography with X-ray or electron beam, it is necessary to conduct a defect inspection by irradiating an electron beam thereon. However, in the case that a diamond film is obtained by the above CVD methods of which source gas is a hydrogen-diluted carbon contained gas, it suffers from a problem that since its electric resistance is high as shown the above-described range, charged particles are stored therein, so that a charge-up phenomenon is easily generated and the defect inspection is not conducted promptly and precisely.
Besides, in the case that a diamond film is actually used for a mask membrane for lithography with electron beam, if its electric resistance is high, a charge-up phenomenon is also generated, so that it is difficult to conduct a high-precision transcription.
In order to reduce an electric resistance of a diamond film for avoiding such a charge-up phenomenon, it is suggested that when forming a diamond film, a dopant gas such as diborane (B2H6) or phosphine (PH3) is introduced into a reaction container, and vapor phase reaction is conducted by using the dopant gas and the aforementioned hydrogen-diluted carbon contained gas.
Specifically, it was reported that in the case that vapor phase reaction is conducted by introducing a hydrogen-diluted methane gas containing diborane as a dopant into a reaction container so that a P-type diamond film is formed, electric resistance of the diamond film can be reduced to 10−2 Ω·cm (K. Marumoto, J. Appli. Phys., 31 (1992) 4205-4209).
However, in the case that diborane is used as a dopant source of boron, its threshold limit value is 0.1 ppm and phosphine is used thereas, it is 0.3 ppm, so that there is anxious about effect on the human body. Namely, in the case of a doping with these gases, any leakage is not permitted, and thus an additional safety device or the like must be provided, so that too much cost is required. Meanwhile, because of explosive nature of these gases, they must be treated with the greatest care, and an apparatus also must be have measures against the explosive nature. Therefore, it is preferable that a doping with these gases is avoided as much as possible.